Vapor deposition method including specified solid angle of radiant heater

ABSTRACT

A METHOD FOR VAPOR DEPOSITING A MATERIAL ON A SUBSTRATE WHEREIN THE SUBSTRATE IS HEATED BY RADIANT HEATERS MAINTAINED AT TEMPERATURES SUFFICIENTLY HIGH TO PREVENT CONDENSATION OF VAPOR THEREON AND OF A CONFIGURATION WHICH PREVENTS OVERHEATING OF THE SUBSTRATE.

Feb. 2,-1971 KENNEDY 3,560,252

VAPOR DEPOSITION METHOD INCLUDING SPECIFIED SOLID ANGLE 0F RADIANT HEATER Filed Aug. 13, 1968 m A V III/II ///AAAAA4 AA /////l llll 4 3 m 7 7 I 2 3 Q Q m w INVENTOR KURT D. KENNEDY ATTYS.

United States Patent Office Patented Feb. 2, 1971 3,560,252 VAPOR DEPOSITION METHOD INCLUDING SPECIFIED SOLID ANGLE OF RADIANT HEATER Kurt D. Kennedy, Berkeley, Calif., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Aug. 13, 1968, Ser. No. 752,252 Int. Cl. C23c 13/02 U.S. Cl. 117-107 7 Claims ABSTRACT OF THE DISCLOSURE A method for vapor depositing a material on a substrate wherein the substrate is heated by radiant heaters maintained at temperatures sufliciently high to prevent condensation of vapor thereon and of a configuration which prevents overheating of the substrate.

This invention relates to vapor deposition and, more particularly, to an improved method for vapor deposition under conditions of elevated substrate temperatures.

Vapor deposition of materials on substrates is of considerable advantage in many production operations. Vapor deposition is generally carried out in a relatively high vacuum environment in which the material to be deposited is evaporated by raising it to a sufficiently high temperature. The required heat may be produced by suitable heating, such as by electron beams.

For some types of materials, adhesion between the deposit and the substrate, and the quality of the deposit, may be enhanced by carrying out the vapor deposition at elevated substrate temperatures, that is, temperatures above the ambient temperature in the vacuum environment. One of the most convenient ways of heating a substrate is by utilizing some type of radiant heater positioned near the substrate. Where the substrate is a discrete element, such as a small machine part or a turbine blade, it may be necessary to provide a relatively large surface radiator in the radiant heater in order to achieve uniform heating of the part being coated. With a relatively large surface area, the radiator of the radiant heater may be exposed directly to the vapor emanating from the vapor source, since the part being coated may be too small to shield all of the radiator surface. In the event vapors condense on the radiator, serious damage or destruction thereof can result.

In order to prevent condensation of vapor on the radiator of a radiant heater, the radiant heater may be operated so the radiator is at a temperature exceeding the reemission temperature of the evaporant material, that is, the temperature at and above which vapor will not condense on, but will be reflected from, the surface of the material. This temperature varies for different materials and is related to the boiling point of the material. When the temperature of the radiator exceeds the re-emission temperature, however, the temperature of the part being coated may become so high that one or more undesirable occurrences will take place. If the substrate temperature exceeds the melting point of the material of which the substrate is composed, structural damage to the substrate may result. If the temperature of the substrate is raised to equal or to exceed the re-emission temperature of the vapor material, the vapor will not condense on the substrate and no coating will result. Under some circumstances, if the temperature of the substrate exceeds the melting temperature of the deposited material, the coating may run and become uneven.

Accordingly, an object of the invention is to provide an improved method for vapor depositing a material on Cal a substrate in which the temperature of the substrate is maintained above ambient.

Another object of the invention is to provide a method for vapor deposition in which damage to radiant heater elements, used to heat the substrate, is minimized.

A further object of the invention is to provide a method for vapor deposition wherein parts being evaporatively coated may be maintained at uniform temperatures above ambient without overheating, by using radiant heaters operating at temperatures exceeding the re-emission temperature of the vapor material.

Other objects of the invention will become apparent to those skilled in the art from the following detailed description, taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of apparatus constructed in accordance with the invention for performing the method of the invention;

FIG. 2 is a plan view of an alternate embodiment of the invention; and

FIG. 3 is a sectional view taken along the line 33 of FIG. 2.

In accordance with the invention, an evaporable material is deposited on a substrate 11 while the substrate is heated by a radiant heater 12. The radiator means 13 of the radiant heater are maintained at a temperature which exceeds the re-ernission temperature of the deposit material. The substrate is maintained at a temperature which is less than the lesser of the re-emission temperature of the deposit material and the melting temperature of the substrate material. To provide for the maintenance of such temperatures, the radiant heater radiates heat from a solid angle in steradians which does not exceed 41r times the fourth power of the absolute temperature of the sub- I strate divided by the fourth power of the absolute temperature of the radiator means 13.

Referring now more particularly to FIG. 1, a vapor deposition system is illustrated. The system includes a vacuum tight enclosure 14 having a duct 15 through which the interior of the enclosure is evacuated by means of a vacuum pump 17. Typical vapor deposition operations may be carried out in a vacuum of 1 torr or less. A suitable sealable port, not illustrated, may be provided in the wall of the vacuum tight enclosure 14 to permit insertion and removal of parts therein.

Vapor is produced within the vacuum enclosure 14 from molten material 18 contained within a crucible 19. Coolant passages 21 are provided in the walls of the crucible to remove heat therefrom and thus cause a region between the cooled crucible and the molten material to solidify and form a skull 22. The skull 22 helps to maintain the molten material from contact with the material of the crucible walls, preventing any reaction therebetween to prolong the life of the crucible and to maintain the purity of the material contained therein.

The material 18 contained within the crucible 19 is heated by bombarding its surface with one or more electron beams. Only one electron beam 23 is indicated in the drawing to preserve clarity. The electron beam 23 is produced from an electron gun including an emissive filament or emitter 24 comprised of tungsten or other suitable material. Electrical connection, not illustrated, is made to the emitter 24 to heat it to a temperature sufficient to produce a substantial amount of free electrons. The emitter is disposed in a recess in a backing electrode 26 and thus the electrons produced by the emitter are shaped into a beam emanating from the open side of the slot or trough in the backing electrode. Such electrons are accelerated into a beam by means of an accelerating anode 27 maintained at an appropriate accelerating potential. Suitable means, not illustrated, are provided for supplying the potentials and for supporting the elements at the various positions shown. The electron beam gun is positioned below the level of the material 18 in the crucible 19 in order to prevent the emitter 24 and the other elements of the electron beam gun from becoming clogged due to condensation of vapor thereon.

:In order to direct the electron beam 23 from the emitter 24 onto the surface of the molten material 18 in the crucible 19, an electromagnetic field is established over the surface of the molten material. The lines of force of the magnetic field curve over the top of the material to form a barrel shaped electromagnetic field in which electrons passing therethrough are deflected downwardly onto the surface and converged slightly. Such field is established between a pair of pole pieces 28 positioned on opposite sides of the crucible 19 and joined by a low reluctance core 29 near one end. An electromagnetic coil 31 surrounds the core 29 and is energized, by suitable means not shown, to provide a desired polarity between the pole pieces and thus establish the electromagnetic field. An electron beam gun system generally of the type shown and described is shown and described in US. Pat. No. 3,177,535 assigned to the present assignee.

In the illustrated embodiment, the substrate 11 com prises a turbine blade for a jet engine and includes a pair of end flanges 32 disposed parallel with each other and joined by a hollow shaped web or fin 33. The substrate or part 11 being coated is supported in the flow of vapor moving upwardly from the material 18 in the crucible 19 by suitable means, not illustrated. Such supporting means for the substrate 11 also provide rotation of the substrate in order that all its surfaces may be brought into the vapor flow and thus receive an even coating.

Under some circumstances, it is desirable to maintain the substrate temperature substantially above the ambient temperature during the coating process. In order to provide even heat to a discrete substrate, it is usually desirable that the radiant heat input to the substrate be from as large an area surrounding the substrate as possible. To prevent condensation of vapor on the radiators of the radiant heater, however, it is desirable to operate the radiators at temperatures exceeding the re-emission temperature of the vapor material. When this is done, too much radiant heat input to the substrate may result, raising its temperature beyond desirable limits.

In the embodiment of FIG. 1, the radiant heater 12 includes radiator means 13 comprising a /2-inch thick plate of tungsten or molybdenum and which is slightly larger in width and length than the corresponding dimensions of the part being coated. The plate 13 is heated by an electron beam 34 produced in an electron beam gun including an emissive filament or emitter 36, a backing electrode 37 and an accelerating anode 38. Because the emitter 36 is positioned on the opposite side of the plate 13 from the substrate 11 and the evaporating material 18, impingement of vapor on the emitter, and resultant damage thereto, is minimized. By varying the electron beam power, either through suitable variation in the accelerating potential or in the temperature of the emitter, the temperature of the plate 13 may be controlled. The substantial nature of the plate 13 provides a long life radiator which is easy to construct and which does not tend to destroy itself in the presence of surface flaws as do some other types of more delicate resistance heating radiators. There are no electrical leads exposed to the vapor flow such that condensate forming thereon might flake off and fall back into the crucible, disrupting the evaporation process.

The effect of a radiant heater on a discrete substrate, such as the part illustrated in the drawings, may be closely approximated from the total solid angle subtended by the surface or surfaces from which heat is radiated. The solid angle in steradians, at any given point (e.g., the geometric center of a part being coated) subtended by a given surface (e.g., a radiator) is equal to the area of the portion of the surface of a sphere of unit radius, center at the given point, which is cut out by a conical surface originating at a vertex at the given point and passing through the perimeter of the given surface. Assuming a part were completely enclosed in a radiator, a solid angle of 411- steradians would exist. By measuring the included angle from an apex at the geometric center of a part being coated to the extreme edges of the radiating element in two mutually perpendicular planes passing through the geometric center of the part being coated and perpendicular to the extreme edges of the radiating element, and approximate value for the solid angle may be arrived at which is sufficiently accurate for most cases. Thus, assuming the geometric center of the part or substrate 11 in FIG. 1 is at the point 39, the included angle in either the plane of the paper or a plane perpendicular to the paper may be measured between lines drawn from the point 39 to the appropriate opposite edges of the slab 13. The sum of the two measured included angles approximates the solid angle in steradians.

Using this approach, an equilibrium condition may be approximated wherein the heat input to the substrate is equal to the heat losses therefrom. The heat input to a substrate being coated by vapor deposition includes not only any heat supplied thereto from radiant heaters, but the heat supplied thereto from the hot vapor condensing thereon. If the latter factor is ignored, an equilibrium condition may be achieved where the solid angle of the radiator equals, in steradians,

and T equals the absolute temperature of the radiator, E equals the emissivity of the substrate, and E equals the emissivity of the radiator. The two temperatures may be arrived at by considering that the temperature of the radiator should exceed the re-emission temperature of the deposit material, and the temperature of the substrate should be less than the lesser of the re-emission temperature and the substrate melting temperature. It may be that peculiar requirements of the system make it necessary to maintain the substrate at an even lower temperature than that mentioned, such as less than the melting temperature of the deposit material to prevent the deposit material from running on the surface of the substrate. In any event, an approximation of the heater size may be arrived at by calculating the maximum solid angle as above and using that figure to determine the dimensions of the radiating element. The radiating element may be longer in one direction than in the other, depending upon the part shape.

For certain part shapes, a configuration of the radiant heater 12 as shown in FIG. 1 may not provide the desired uniformity in heating of the part. An embodiment of apparatus in accordance with the invention and constructed to provide a more uniform heating of the part is shown in FIGS. 2 and 3. The particular construction therein utilizes a radiant heater comprised of a plurality of strips or wires spaced from each other and approximating the uniformity of a solid radiator, such as in FIG. 1, while maintaining a lower equilibrium temperature of the part being heated. This is possible since the total solid angle is reduced due to the gaps between the strips or wires. The total solid angle is equal to the sum of the solid angles subtended by the respective surfaces of the strips or wires. Uniformity in heating is maintained due to the spacing of the radiating elements.

Referring now more particularly to FIGS. 2 and 3, the part 11 is identical with that in the previous embodiment, including the end flanges 32 connected by the fin or web 33. The radiant heating means 41 is comprised of three arched parallel strips 42 spaced from each other and arched over the top of the part being heated. The strips are supported by a pair of flanged supports 43 and 44 comprised of insulating material to which the strips are bolted. A plurality of conductors 46 are also bolted to the support 43 at one end to form electrical contact with the strips 42. A plurality of similar conductive strips 47 are bolted in electrical connection with the strips 42 to the supports 44. The conductors 46 and 47 extend through the walls 14 of the vacuum enclosure and are electrically connected across a variable current source 48. The strips may be provided with coolant coils 49 in order to prevent Overheating thereof.

The included angle in the plane of the paper, as may be seen from FIG. 3, is almost 180. On the other hand, the total included angle in the plane perpendicular to the 1 paper is very small, because the narrow width of the strips produces several small included angles, the sum of which is small. Accordingly, the approximated total solid angle (arrived at by adding the sum of the included angles in the plane perpendicular to the paper in FIG. 3 to the included angle of 1r in the plane of the paper) is small, and may be made sufliciently low to prevent overtungsten may reach well over 2,000 E, preventing condensation thereon, whereas the part may achieve equilibrium operation at approximately 1,000 F.

The following table lists some further illustrative conditions for which equilibrium heating may be achieved, at least theoretically or at very low evaporation rates. As previously mentioned, additional heat input is provided to the part being coated from the vapor condensing thereon. When evaporation rates are sufiicient as to make this additional heat input significant, it must be taken into account. Accordingly, the actual solid angle will have to be less than the theoretical solid angle given in the table by an appropriate amount. This may be achieved approximately by calculating the ratio of the heat added by the radiators alone to the total heat added (by the radiators plus the vapor) and multiplying the calculated ratio by the theoretical solid angle such as the table. Thus:

where H and H, are the heat in calories added to the part by the radiators and the vapors respectively.

Actual solid angle=theoretical solid angle Element Solid Element Part number angle, tempertemper- Substrate material Coating material and size steradians ature, K ature, K

Copper (e=.08) Aluminum,e=. SO" x 10) 134 1, 850 773 Iron, 6=.25 Cobalt, s=.26. 1 x 10") 4. 25 2,175 1, 675 Iron E=.22 Silver, e=.24 (1 x 2. 81 1, 600 1,130 Nickel, e=.25 Chromium, e= 5 10(% x 12) 5.12 2, 000 1, 600 Aluminum, =.035 Bismuth, e=.18. 3(% x 9) .0329 1,175 400 Titanium, 6 12-58 Manganese, E=1. 10(1 x 10") 3. 98 2,000 1, 500

heating of the substrate, while permitting heating of the individual strips 42 to the required level.

By way of example, if the part 11 is to be operated at 1800 F. and is to be coated with a metal such as iron, aluminum, cobalt, or nickel, and if the part 11-is about 3 inches in its maximum dimension, three tantalum strips 42 may be used, about inch wide, mils thick, and 12 inches long, being spaced about 2 inches apart. If a heating current of 450 amps is passed through each strip, the strips may be raised to a temperature exceeding 2,000 F. At this temperature, metal such as iron, aluminum, cobalt or nickel will not coat the heating strips. Under the foregoing conditions, the part may reach an equilibrium temperature of approximately 1,000 F. Vapor that misses the part and strikes the heated strips may be reflected back onto the part, thus increasing the vapor collection efliciency of the system.

A configuration constructed in accordance with that illustrated in FIGS. 2 and 3 and utilizing three tantalum strips /2 inch wide, 15 mils thick and 12 inches long, spaced 2 inches apart is capable of creating a hot Zone between 1350 with 240 amps to 1850 with 300 amps. Currents below 240 amps lead to coating by iron, aluminum, etc., of the tantalum strips and rapid failure by alloying. Currents above 300 amps lead to failure of the strips by overheating. The particular desired temperature of the heating element may be selected by following the general rule that vapors will not condense on surfaces at temperatures corresponding to a vapor pressure (of the condensed vapor) greater than the partial pressure of the vapor adjacent to the surface. Exceptions to this rule occur, however, when unusual misfits exhibit between the vapor atom (or molecule) and the substrate crystal structure. These latter situations are discussed extensively in Dushman, Vacuum Technique.

Under some circumstances, it may be desired to produce extremely high temperatures in the heating elements because of the particular high re-emission temperature of the material being deposited. This being the case, and for a part 11 of about 3 inches maximum dimension, the strip type heating elements illustrated in FIGS. 2 and 3 may be replaced with three 60-mil tungsten wires 12 inches long and spaced 2 inches apart and arched over the top of the part as illustrated in FIG. 3. If the tungsten wires are heated with 100 amps, the surface temperature of the 75 It may therefore be seen that the invention provides an improved method and apparatus for vapor depositing a vapor material on a substate. The substrate is heated during the deposition operation by radiant heaters which avoid damage due to condensation thereon and yet avoid overheating of the part being coated.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings.

Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A method of vapor depositing a material on a substrate, comprising: positioning a source of material and the substrate within a vacuum environment and heating the source of material to cause vapor therefrom to impinge upon the substrate, heating the substrate by radiated heat, and maintaining the radiant heat source at a temperature which exceeds the re-emission temperature of the deposit material while maintaining the substrate at a temperature which is less than the lesser of the re-emission temperature and the substrate melting temperature by radiating heat from a total solid angle in steradians of no greater than 41l' times the fourth power of the absolute temperature of the substrate times the emissivity of the substrate divided by the fourth power of the absolute temperature of the source of radiated heat times the emissivity of the radiator.

2. A- method according to claim 1 wherein the substrate temperature is maintained at less than the melting temperature of the deposit material.

3. A method according to claim 1 wherein the substrate is rotated with respect to the vapor source and the radiant heat source.

4. A method in accordance with claim 1 wherein the substrate is heated by means of electron beam bombardment.

5. A method in accordance with claim 1 wherein the vapor source is heated by means of electron beam bombardment.

6. A method in accordance with claim 5 wherein the substrate is a turbine blade.

7. A method in accordance with claim 5 wherein the vapor source is a member selected from the group con- 7 8 sisting of iron, aluminum, cobalt, nickel, silver, chromium, 3,427,154 2/1969 Mader et a1. 117-1 07X bismuth and manganese 3,470,018 9/1969 Smith et a1. 117-106X 3,492,152 1/1970 Cariou et a1. 117106X References Cited UNITED STATES PATENTS 5 MORRIS KAPLAN, Primary Examiner 3,312,572 4/1967 Norton et al. 11849X CL 3,392,056 7/1968 Maskalick 117107X 3,400,014 9/1968 Blumberg et a1. 117-107X 1'17106; 11849.5 

